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961.
The ancient Maya subsisted in an environment limited by shallow soils and unpredictable weather patterns until their collapse ∼A.D. 800–900. Ancient subsistence can be a difficult subject, with little physical evidence of agricultural artifacts and structures. This study characterized soil profiles and utilized changes in stable carbon isotope ratios of soil organic matter (SOM) to locate and interpret areas of ancient C4 plant growth and maize (Zea mays) cultivation among the Maya. The investigation indicated some of the challenges the Maya faced, including shallow and sloped soils in some areas. The C4 plant signature was found in seasonal wetland soils on the opposite side of the Laguneta Aguateca from the ruins of Aguateca, but not in the perennial wetlands on the immediate side. No C4 plant signature was detected in the shoulder and backslope soils. Based on these findings, the ancient Maya of Aguateca probably adapted to their environment by farming rich toeslope soils. It is possible that maize was also grown in the seasonal wetlands adjacent to the site. If the steep backslope soils around Aguateca were used in ancient agriculture, the evidence has probably eroded away. © 2007 Wiley Periodicals, Inc. 相似文献
962.
963.
Ryan Ekre Paul C. Johnson Bruce E. Rittmann Rosa Krajmalnik‐Brown 《Ground Water Monitoring & Remediation》2014,34(2):60-70
This work focuses on the site‐specific assessment of source zone natural attenuation (SZNA) at chlorinated aliphatic hydrocarbon (CAH)‐impacted sites. The approach is similar in some ways, but different in other ways from recently proposed SZNA assessment paradigms for petroleum‐impacted sites. The similarities lie in the organization of the approach around determining: (1) whether or not SZNA is occurring, (2) the current SZNA rate, and (3) what is the future projection for SZNA rate changes and the final state of the source zone. Differences lie in how those rates are determined, especially with respect to the quantities measured and data reduction. Petroleum‐impacted site SZNA approaches emphasize quantifying fluxes of electron acceptors, while the proposed CAH assessment approach emphasizes quantifying parent and daughter compound fluxes. A paradigm for assessing SZNA at CAH sites is presented and its use is illustrated, for example former dry cleaner site, where the SZNA rate was approximately 3.5 kg/year as tetrachloroethylene (PCE) with about 80% of the mass loss attributed to groundwater flow and 20% attributed to vapor transport. 相似文献
964.
Lane R. Johnson 《Pure and Applied Geophysics》2014,171(8):1641-1668
At The Geysers geothermal reservoir in northern California, evidence strongly suggests that activities associated with production of electric power cause an increase in the number of small earthquakes. First-degree dynamic moment tensors are used to investigate the relationship between induced earthquakes and injection of water into a well as part of a controlled experiment in the northwest Geysers. The estimation of dynamic moment tensors in the complex shallow crust at The Geysers is challenging, so the method is described in detail with particular attention given to the uncertainty in the results. For seismic events in the moment magnitude range of 0.9–2.8, spectral moduli of dynamic moment tensors are reliably recovered in the frequency range of 1–100 Hz, but uncertainty in the associated spectral phases limits their use to a few simple results. A number of different static moment tensors are investigated, with the preferred one obtained from parameters of a model fitted to the spectral modulus of the dynamic moment tensor. Moment tensors estimated for a group of 20 earthquakes exhibit a range of source mechanisms, with over half having significant isotropic parts of either positive or negative sign. Corner frequencies of the isotropic part of the moment tensor are about 40 % larger than the average of the deviatoric moment tensor. Some spatial patterns are present in source mechanisms, with earthquakes closely related in space tending to have similar mechanisms, but at the same time, some nearby earthquakes have very different mechanisms. Tensional axes of displacement in the source regions are primarily horizontal, while the pressure axes range from near horizontal to vertical. Injection of water into the well in the center of the study area clearly causes an increase in the number of earthquakes per day, but an effect upon source mechanisms is not evident. 相似文献
965.
中国黄土高原西缘甘肃万象洞MIS 5石笋δ18O记录与南方地区石笋记录的对比研究 总被引:1,自引:0,他引:1
位于青藏高原与黄土高原过渡带的甘肃武都万象洞石笋WXSM51和WXSM52提供了MIS 5(118~79 kaB.P.)高分辨率的δ18O记录.研究表明,万象洞石笋δ18O值与夏季风强度呈负相关关系,与我国西南部的贵州董歌洞石笋δ18O记录有良好的对应关系,并与高纬度的格陵兰NGRIP冰芯δ18O记录和65°N太阳辐射强度有很好的一致性,说明万象洞石笋δ18O记录了118~79 kaB.P.期间亚洲季风强度的变化,同时也说明东亚季风强度的变化和全球气候变化同步,而且主要受控于北半球太阳辐射强度的变化.同时它与地中海碳酸盐记录有很好的相似性,和巴西石笋δ18O记录在千年尺度上表现出相反的变化趋势,说明东亚季风区、地中海地区以及巴西季风区之间存在密切的联系,指示了南北半球气候在千年尺度上存在"跷跷板"(seesaw)现象.万象洞石笋δ18O记录的MIS 5b与MIS 5a突发性转换,与NGRIP冰芯δ18O记录相似,而与神农架记录存在差异,说明万象洞地区对亚洲季风强度的响应更为敏感. 相似文献
966.
The validation and subsequent application of the current three-dimensional numerical hydrodynamic model of Chesapeake Bay is presented. The numerical model solves conservation equations for water mass, momentum, salinity, and heat on a boundary-fitted grid in the horizontal plane and a Cartesian z-grid in the vertical. A generalized ADI finite difference scheme is employed in conjunction with mode splitting technique, solving external and the internal modes. The 10-year boundary conditions including tide, slinity, temperature, wind, heat exchange coefficient, river and non-point source flows were constructed. Model validation was accomplished by demonstrating the model's ability to reproduce observed data over time scales ranging from tidal to seasonal periods. The major parameters compared include tidal elevation, intra-tidal and residual velocities, salinity, temperature, stratification, and flux calculated through the Bay mouth.After validation, the model was applied to simulate bay hydrodynamics for the 10 years of 1985–94. These results were used to drive the three-dimensional water quality model of Chesapeake Bay, which is discussed in a companion paper. 相似文献
967.
Adrian G. Johnson 《The Australian geographer》2000,31(2):209-235
This study reconstructs environmental conditions at Mill Creek, within the lower Hawkesbury-Nepean Valley, by the use of fine resolution palaeoecological studies of sediments. Archaeological surveys and historical accounts are used to investigate ecosystem response to known human activities. Research found that during prehistoric times (between 820 BP and the 1790s), the study area was well vegetated with dry sclerophyll communities on the valley sides, and a highly productive wetland community occupied the moist valley floor. Valley sides were generally stable, and little fire was evident in the landscape. Sediment was mainly deposited on the valley floor from the Hawkesbury-Nepean River during flood. After European settlement in the 1790s, sedimentation rates and charcoal preserved within sediments increased significantly, probably due to agricultural and clearing practices, both locally and upstream. Consequently, vegetation communities were altered. In 1967, the study area became part of Dharug National Park. Forestry and agricultural activities were excluded from the catchment, and the fire regime was reduced in frequency. Present-day vegetation communities appear to be becoming more like those that prevailed during prehistoric times. However, sedimentation rates remain elevated due to continued disturbance to the greater Hawkesbury River catchment. Recent poor water quality, coupled with a succession of floods on the Hawkesbury River, has increased wetland productivity levels, and their spatial extent, at Mill Creek. 相似文献
968.
969.
Partitioning Tracer Tests for Evaluating Remediation Performance 总被引:1,自引:0,他引:1
R. Brent Cain Gwynn R. Johnson John E. McCray William J. Blanford Mark L. Brusseau 《Ground water》2000,38(5):752-761
970.
Arne Krtzinger Ludger Mintrop Douglas W. R. Wallace Kenneth M. Johnson Craig Neill Bronte Tilbrook Philip Towler Hisayuki Y. Inoue Masao Ishii Gary Shaffer Rodrigo F. Torres Saavedra Eiji Ohtaki Eiji Yamashita Alain Poisson Christian Brunet Bernard Schauer Catherine Goyet Greg Eischeid 《Marine Chemistry》2000,72(2-4)
The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements. 相似文献